1. Field of the Invention
The present invention relates to a laser irradiation method and more particularly to a laser irradiation method for controlling laser irradiation to an irradiation object by an autofocusing mechanism. Moreover, the present invention relates to a method for manufacturing a semiconductor device with the use of the laser irradiation method.
2. Related Art
In recent years, a technique to manufacture a thin film transistor (TFT) over a substrate has made a great progress, and application development to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous crystal semiconductor film, and therefore high-speed operation is possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, it has been tried to control a pixel by a driver circuit formed over the same substrate as the pixel, which has been conventionally controlled by a driver circuit provided outside the substrate.
A substrate used in a semiconductor device is expected to be a glass substrate in terms of cost. However, the glass substrate is inferior in heat resistance and easy to change in shape due to the heat. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over the glass substrate, laser annealing is employed to crystallize a semiconductor film formed over the glass substrate in order to prevent the glass substrate from changing in shape due to the heat.
Compared with another annealing method which uses radiant heat or conductive heat, the laser annealing has advantages that the processing time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally.
In general terms, the laser annealing to the semiconductor film is often performed by using an excimer laser. The excimer laser has advantages of its high output power and high repetition rate. Moreover, the laser beam emitted from the excimer laser has an advantage that it is sufficiently absorbed in a silicon film, which is often employed as the semiconductor film. In the laser irradiating step, a beam spot of the laser beam on the irradiation object is shaped into a linear spot (including rectangular and elliptical spots) by an optical system, and the beam spot is moved relative to the irradiation object in a short-side direction of the linear beam spot. By such laser irradiation, the laser annealing can be performed to the irradiation object effectively.
Moreover, a continuous wave laser (also referred to as a CW laser) can be used in the laser annealing step. When a laser beam emitted from the CW laser is shaped into a linear spot and the semiconductor film, which is the irradiation object, is moved relatively in the short-side direction of the beam spot on the irradiation object, a large crystal grain extending long in the moving direction can be formed in the semiconductor film. A TFT manufactured in accordance with the extending direction of the large crystal grain can have higher carrier-mobility than a TFT manufactured using the excimer laser. With the TFT having high carrier-mobility, the circuit can be driven at higher speed, and therefore a driver, a CPU, and the like can be manufactured.
The laser beam emitted from the CW laser to be generally used in the laser annealing has a wavelength of 532 nm because this wavelength is sufficiently absorbed in amorphous silicon (a-Si) and the conversion efficiency from the fundamental wave by the non-linear optical element is high. Usually, the shorter the wavelength of the laser beam is, the more a-Si absorbs the laser beam. Meanwhile, the shorter the wavelength is, the lower the power of the laser beam is.
A technique for forming the TFT with the use of the semiconductor film crystallized by the above method has been carried out in many fields.
When the power of the laser beam is low, the laser beam is condensed on one point in the irradiation object by a lens in order to increase the energy density or the power density of the laser beam. Moreover, even in the case of forming a pattern on the irradiation object directly by irradiating the irradiation object with the laser beam, the beam spot is condensed on the irradiation object by the lens. For example, when the semiconductor film is crystallized using the CW laser, the beam spot is shaped into an elongate spot such as a rectangular, elliptical, or linear spot on the irradiation object and condensed to have a length of several μm in the short-side direction by the lens in order to increase the throughput as much as possible. Furthermore, when a fine pattern is imaged directly to the irradiation object by the laser irradiation, the beam spot is narrowed further.
To narrow the diameter of the beam spot formed over the irradiation object by condensing the laser beam, it is necessary to use a lens having large numeral aperture (NA). Generally, NA and a focal depth Z satisfy the equation Z=±λ/2NA2 where λ is the wavelength of the laser beam. Therefore, when the lens has larger NA, the focal depth of the lens becomes shorter accordingly. For example, when using the CW laser, the focal depth needs to be adjusted to be approximately several μm.
However, when a substrate typified by a glass substrate becomes larger, the variation of the thickness of the substrate becomes more remarkable. The thickness may vary within the substrate by several tens μm. For example, when the semiconductor film formed over the glass substrate or the like whose thickness is not even is annealed by the laser irradiation, the distance between the lens and the irradiation object depends on the location in the substrate, and the beam spot shape changes depending on the location accordingly. For this reason, the crystallinity differs depending on the location even in the semiconductor film formed over the same substrate.